The present invention relates to nonvolatile memories and, in particular, to memories based on magnetoresistance.
Conventional nonvolatile memories—semiconductor as well as magnetic RAM (MRAM)—require shielding to protect against radiation. Conventional MRAM architectures provide nonvolatility by using magnetic storage elements, but employ CMOS transistors to address individual memory cells, thereby incorporating vulnerability to radiation damage into the system.
In recent years, the MRAM industry has largely focused on developing and commercializing nonvolatile memories based on magnetic tunnel junction (MTJ) structures. Both conventional MTJ memories as well as the more recently developed “toggle” MTJ memories have drawbacks which present obstacles to achieving the kind of density considered acceptable for commercialization. First, the minimum cell size for MTJ memories is significantly larger than the CMOS processing linewidth because of the complexity of the cell design and the interference of closely spaced cells.
Second, the overlap between the switching-field distributions of full-selected and half-selected cells in MTJ designs result in unacceptably high write error rates as the distance between cells shrinks, which is likely due to the demagnetizing fields from adjacent cells. Finally, most MTJ designs also suffer from a problem known as “magnetization creep” in which the cells become demagnetized over time so their information contents are lost.